It was unprecedented. In 2017, astronomers discovered the first known interstellar object in our Solar System: 'Oumuamua, a mysterious cigar-shaped enigma, identified as our first visitor from outer, outer space. But just because 'Oumuamua was the first detected interstellar object, doesn't mean it was the first ever. Just five years ago, in fact, Earth's atmosphere was struck by something that may have originated far outside our own Solar System -- and we never even realized it.